Petrogenesis of pyroxene–oxide intergrowths from kimberlite and cumulate rocks: co-precipitation or exsolution?

Pyroxene–oxide intergrowths occur in rocks representing equilibration over a wide range of P–T–f_O2 conditions; these intergrowths have three modes of occurrence: (1) nodular pyroxene–ilmenite intergrowths from kimberlite, (2) “myrmekitic” pyroxene–oxide intergrowths from cumulate rocks and mantle xenohths, and (3) fine lamellae of spinel or ilmenite in pyroxenes from cumulate rocks and mantle peridotites. Examination of three nodular clinopyroxene–ihnenite intergrowths and an ilmenite–spinel websterite from the Elliott County kimberlite, Kentucky and a cumulate norite from a small gabbroic pluton from Chester County, South Carolina has revealed intergrowths representing all three modes of occurrence. The nodular clinopyroxene–ilmenite intergrowths are interpreted to have formed during co-precipitation of Cr-poor clinopyroxene (73.0–76.8 mol.% Di) and picroilmenite (44.7–46.6 mol.% MgTiO₃ and 5.4–5.8 mol.% Fe₂O₃) at 1295°–1335°C from the “proto-kimberlitic” hquid which produced the Elliott County kimberlite megacryst suite. The Elliott County websterite xenolith contains fine (1–5 μm) lamellae of magnetite (12.6 mol.% (Fe,Mg)Al₂O₄ and 8.0 mol.% (Fe,Mg)₂TiO₄) and pleonaste (6.8 mol.% (Fe,Mg)Fe₂O₄ and 0.1 mol.% (Fe,Mg)₂TiO₄) within aluminous clinopyroxene (8.5 mol.% CaTs and 7.3 mol.% Jd) and orthopyroxene (7.8 mol.% MgTs); these intergrowths are interpreted to have formed during “exsolution” of spinel from high-temperature (1105°–1140°C), non-stoichiometric pyroxenes. Coexisting oxides suggest equilibration at 695°C and f_O2 = 10^−13.8. The Chester cumulate norite contains both ilmenite lamellae (11.6 mol.% MgTi0₃ and 5.7 mol.% Fe₂O₃) in orthopyroxene (4.4 mol.% MgTs) and clinopyroxene (5.6 mol.% CaTs and 2.7 mol.% Jd) and myrmekitic intergrowths of orthopyroxene and magnetite (0.8 mol.% (Fe,Mg)Al₂O₄ and 0.5 mol.% (Fe,Mg)₂TiO₄) and minor ilmenite (14.3 mol.% MgTiO₃ and 3.8 mol.% Fe₂O₃). The lamellar intergrowths were produced during “exsolution” from high-temperature (1200°C), non-stoichiometric pyroxenes. Myrmekitic intergrowths formed during co-precipitation of orthopyroxene, magnetite, and ilmenite. Coexisting Fe–Ti oxides suggest equilibration temperatures of 480°–515°C at f_O2 = 10^−21.0−10^−23.7.